Gordon Brown is trying to head off the biggest backbench protest of his prime ministership by offering to set up an independent commission to consider the rights of a million agency and temporary workers.

The commission would bring together the TUC and CBI in a one-off inquiry to see how such workers could be given pay and conditions comparable with those of permanent workers.

Ministers see it as the best way to resolve probably the biggest outstanding workplace issue facing "vulnerable workers" in Britain. They have already held talks with both the TUC and CBI.

The commission would seek to arbitrate on all the contested issues surrounding agency workers, including length of employment before any new rights would be enforceable. It would set out a process for identifying those full-time workers with whom an agency worker could compare wages and conditions. It would also examine how the impact of any voluntary agreement reached in the UK would work under subsequent EU directives governing agency employment.

It is not envisaged that the commission would be a permanent body, but parallels are being drawn with the Low Pay Commission, responsible for setting the minimum wage. The TUC, and an army of senior Labour backbenchers, insist the government must act against the casualisation of the UK workforce.

Ministers are also aware that France is likely to revive a proposed EU directive which could suggest that rights to equal treatment begin after six days in employment. A previous push on the issue in December was headed off by Britain, but France takes over the presidency of the EU later this year.

The CBI is suggesting that there should be a one-year period for temporary workers to qualify, an approach that the TUC believes would leave 900,000 agency workers outside the law. Many Labour MPs insist the issue is a test case of Brown's willingness to mark a break with "Blairite" labour market flexibility policies.

TUC leaders are due to meet the business secretary, John Hutton, to discuss the government proposal further, but many union leaders are sceptical and will want clear terms of reference before agreeing to any commission. The British Chambers of Commerce underlined the stakes yesterday, saying the UK had "benefited from the flexibility" provided by temporary workers. It added the flexible labour market "has been one of the main reasons the economy has created so many jobs. This flexibility must be maintained."

A bill sponsored by Andrew Miller MP, giving rights to agency workers, is due to receive its second reading, with more than 100 Labour MPs expected to attend a debate on Friday week to keep it alive. Miller is keen to give ministers, unions and business time to discuss the commission.